t27b.py: read lane receipts from lab 2 -- the lab URL comes from T27B_LAB #9107

Lab 2 (t27b-lab-2-production) now serves lane requests first (T27_LAB_ROLE=requests_first, see #9026). It turns a request into a receipt in a median of about 38 min, against 70 on lab 1. scripts/tri_loop/t27b.py pins LAB to lab 1, so tri t27b runs and ratchet --run cannot read a lane receipt that lab 2 produced. Base and head of a compare must come from the same lab.

User Scenarios

• Given T27B_LAB=https://t27b-lab-2-production.up.railway.app, when tri t27b runs reads runs/<sha>.json, then it reads lab 2's run for that sha.
• Given no T27B_LAB, when any tri t27b command runs, then it reads lab 1 as before.

Requirements

• FR-001: LAB MUST default to lab 1 and MUST be overridable by the environment variable T27B_LAB.
• FR-002: The change MUST be net 0 hand-written lines (policy: hand-written code in any language but t27 may only shrink (Closes #8227) #8236): one line replaced.

Success Criteria

• grep -c 'T27B_LAB' scripts/tri_loop/t27b.py prints 1.
• git diff --numstat of the PR shows 1 1 scripts/tri_loop/t27b.py.

Boundary

• scripts/tri_loop/t27b.py